Here's a place where I can post my thoughts on new papers, provide updates on my projects, and post info that will eventually be on my website The Theropod Database - http://theropoddatabase.com/ . It will center on theropods, but may delve into other topics as well such as phylogenetics.

Saturday, November 8, 2014

SVP 2014 Day 3- drepanosaurs and sauropods

Okay, it's become apparent this day by day format is flawed because similar talks are grouped together at SVP. This year, basically all of the interesting theropod talks/posters were on Wednesday and Saturday. So tomorrow's post should be huge, but here's a couple interesting of ones from today.

Pritchard and Nesbitt report on a 3D drepanosaur braincase from the Ghost Ranch quarry. Many of its characters "are surprisingly plesiomorphic (e.g., squamosal with massive descending process, quadrate lacking posterior concavity, occipital condyle with notochordal pit), sharing more in common with non-saurian diapsids than early archosauromorphs." "A phylogenetic analysis of 300 characters and 40 early diapsids supports the hypothesis that drepanosaurs fall outside of Sauria." Wouldn't it be funny if pterosaurs were related to drepanosaurs, and thus non-saurians convergent on archosaurs? Even crazier than them being lizards. Also relevant is Paul's (2002) idea Protoavis' type braincase is from a drepanosaur, but Protoavis lacks a ventral squamosal process or a notochordal pit, though its quadrates are straight to slightly concave. So the braincase still seems closest to derived theropods.

Madzia and Borsuk-Bialynicka have a talk titled "New sauropod material from the Nemegt Formation supports the conspecificity of Opisthocoelicauda skarzynskii and Nemegtosaurus mongoliensis." That's an idea Paul supported in his 'field guide', but has generally been considered unlikely to undemonstrable given known remains- a skull for Nemegtosaurus and skeleton lacking neck and skull for Opisthocoelicaudia. Alas, this presentation only says two caudal centra and five pedal unguals were found in the Polish-Mongolian expedition collections, from three different sites in the Nemegt. The unguals "strongly resemble those of O. skarzynskii in their crescent-shaped morphology, great bilateral flattening, and generally subtriangular cross-sections adjacent to their proximal articular surfaces", while the centra "are slightly opisthocoelous and subcircular in cross-section." That's great, but are other titanosaur unguals different? Nor is Opisthocoelicaudia the only titanosaur with opisthocoelous caudals. See Borealosaurus for instance. They say "Since the postcranial elements provide no evidence for the presence of more than one titanosaur in the mid- Maastrichtian of the Nemegt Formation, there is no reason to assume that the type of N. mongoliensis belongs to a different species." This is highly flawed reasoning. We have at best three caudal specimens including the Opisthocoelicauda holotype. The likelihood all three are from one species if two species lived there is pretty high, 25% if my vestigial stat skills are correct (if they were equally abundant, equally likely to be fossilized there, etc.). Just look at another case of large Nemegt dinosaurs- Deinocheirus and Tarbosaurus. The ZPAL has 35 Tarbosaurus but only one Deinocheirus (well, none now since it was moved to the IGM). Even if all of those caudals and unguals are diagnostically Opisthocoelicaudia and from different individuals, that's still only 8:1 for Opistho vs. Nemegt compared to 35:1 for Tarbo vs. Deino.

6 comments:

It was moved to the MPC. The name "IGM" only exists in the AMNH papers, whereas all other resources, including the resources in which the name change was first pointed out, indicate that the old Mongolian Geological Institute (various abbreviations including: GI, GIN, IGM) was formalized to "Mongolian Paleontological Center" (or MPC) for the purposes of not interfering with the name of another museum.

Hi Mickey,I write my response in two posts because Blogspot doesn't allow me to publish the text as a whole. It seems that it also prefers keeping things separate rather then merged into one :P

I'm surprised and super happy at the same time that my poster (not a talk) gained so much attention. I didn't expect it :) This is the first comment that explicitly disagrees with my conclusions (saying that, given the data we have, Nemegtosaurus and Opisthocoelicaudia are most likely synonymous). But that's OK. As I told to everybody with whom I was discussing this during the conference (too bad you weren’t there), the goal of this poster was to force people to discuss the issue and take it into account in their papers. And I seem to be successful (at least with the first part). :) If I’m proven wrong in my interpretation of the sauropod material from the Nemegt Formation, I'll be perfectly satisfied as well :)

When I started my MSc studies in Warsaw, I wanted to look at "nemegtosaurids" and try to add something new to our knowledge of the interrelationships of advanced titanosaurs. However, this was later changed to the question of the possible synonymy of Nemegtosaurus and Opisthocoelicaudia. I wasn't very satisfied with that as I felt that the most important things were already discussed in the literature. And since the most often conclusion was that we simply need an articulated specimen that includes the Nemegtosaurus-type skull with the Opisthocoelicaudia-type postcranial skeleton, I didn't expect that my thesis will actually change anything. As the type of N. mongoliensis is at the collections of the Polish Academy of Sciences, I had also access to the other dinosaurs from Mongolia, and I noticed that there is additional seven sauropod specimens from different localities of the Nemegt Formation. The unguals and the very slightly opisthocoelous mid-caudals are indistinguishable from the type of O. skarzynskii.

And then there is the skull. Nowiński (1971) described N. mongoliensis as a Dicraeosaurus/Diplodocus-like sauropod and for many years it was viewed as such. But the papers from the late nineties and on show that it's a typical titanosaur. Though, its ties to other taxa are often very weakly supported.

Meanwhile, Currie et al. (2003) writes about a series of opisthocoelous caudal vertebrae and footprints from the type locality of N. mongoliensis that they refer to Opisthocoelicaudia.

So, every single fossil from the Nemegt Formation belonging to sauropod postcranial skeleton, mentioned after the description of Nemegtosaurus and Opisthocoelicaudia, suggest that there is only one sauropod.

Is my reasoning really "highly flawed" when I say that I see no reason for keeping these taxa separate? In other words, is the skull really so special that the assumption that it fits the skeleton of Opisthocoelicaudia is that improbable?

I'm very well aware of the existence of Borealosaurus (and another titanosaur that I was told about during the conference and that could be much more important for the Nemegtosaurus-Opisthocoelicaudia problem than my seven isolated fossils) :) I'm also well aware of the fact that several different sauropods might be present at the same environment. Likewise, I know that it would be much better if we also know more about the holotype of Quaesitosaurus orientalis (I've seen it while I was at the RAS paleo museum but it was behind a glass and I couldn’t actually examine anything), the undescribed sauropod dorsals from Gobi (also at the museum; I didn't get permission to study it), or the second skull that is supposedly referable to N. mongoliensis (Maryańska 2000).

The fact that we don't know as much as we want to, however, doesn't mean that we should overlook the issue of the probable conspecificity of Nemegtosaurus and Opisthocoelicaudia. ;)

Oh, and I don’t understand how the Tarbosaurus-Deinocheirus fossils ratio can be an argument. I think that I wouldn’t be surprised if somebody talks about it when Nemegtosaurus is still considered diplodocoid. But it’s not. It’s a derived titanosaur. By the way, when both OTUs are merged, the “new” Nemegtosaurus is inferred at the position previously occupied by Opisthocoelicaudia (and the tree topology is almost unchanged while compared to the “traditional” interpretation of these taxa).

Anyway, my MS on the topic is already written and since I prefer to include the discussion of as much arguments as possible, I would be very happy if you were interested in reading it. I respect your opinions and it might help me to clear up my points. So, let me know if can I send it to you.

RefsCurrie, P. J., Badamgarav, D., and Koppelhus, E. B. 2003. The First Late Cretaceous Footprints from the Nemegt Locality in the Gobi of Mongolia. Ichnos 10: 1–12.

Maryańska, T. 2000. Sauropods from Mongolia and the former Soviet Union. In Benton, M. J., Shishkin, M. A., Unwin, D. M., and Kurochkin, E. N. (eds): The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press, 456–461, Cambridge.

Hi Daniel, thanks for the reply. I agree they could be synonymous, I just don't think that two caudals and five unguals makes it much more likely.

The first issue, which you probably address in your MS, is how diagnostic Opisthocoelicaudia's caudal centra and pedal unguals are. Can the isolated centra and unguals be distinguished from those of other titanosaurs? Besides vulcanodontids, I don't think I've ever read a suggestion that sauropod pedal unguals can be referred to a particular clade, let alone a particular genus. Caudal centra seem a bit more likely to be diagnostic, but also vary within the tail. There are several titanosaur genera that are based on the entire caudal vertebra and which are considered undiagnostic (Titanosaurus, Iuticosaurus, etc.), though I could imagine Opisthocoelicaudia having a unique combination of caudal central characters. The caudal series and skull you mention seem much more likely to be diagnostic once they're described. But Kurzanov (1981) originally thought he had a new Nemegtosaurus skull too, and it ended up being distinct and described as Quaesitosaurus. So I don't put much weight in unjustified referrals.

The second issue is the statistical one. Even if all seven specimens are Opisthocoelicaudia, how many Opisthocoelicaudia specimens do we need before we can say there probably isn't another kind of Nemegt titanosaur postcranium out there that hasn't been found yet? I'd say more than seven, though I admit that's just my gut feeling and not an objective measure. That's what the Tarbo:Deino ratio was supposed to illustrate- if we can find 35 Tarbosaurus' and only 1 Deinocheirus, surely it's not a stretch to find 8 Opisthocoelicaudia's and only 1 Nemegtosaurus. I didn't know about the second reported Nemegtosaurus skull though, and if that's correctly identified, I think it strongly supports your case. If every skull found is Nemegtosaurus and every postcranium found is Opisthocoelicaudia, that becomes too coincidental to ignore, even with relatively few specimens. I don't think 2 skulls are enough to make it probable, but I'd be convinced at 4 skulls and 4 postcrania. I think that'd be a 1.5% chance of being a coincidence.

And yeah, I'd love to see your MS. Just send it to Mickey_Mortimer @ msn.com . Also that's cool that you tried combining the OTUs. The important metric there would be how much longer that tree is than when they're kept separate.